6. JET FORMATION

The processes by which the jets are
accelerated and collimated are still not clearly understood, but it is
believed that several of the concepts proposed for extragalactic jets
can be extended to galactic jets.

Blandford & Znajek
(1977)
take advantage of the fact that, in principle, it is possible to extract
energy and angular momentum from a rotating black hole
(Penrose 1969),
to produce electric and magnetic fields and possibly fast outflowing jets.
A magnetized accretion disk around the Kerr black hole brakes it
electromagnetically. However,
Ghosh & Abramowicz
(1997),
Livio et al (1997)
have called into question that the Blandford-Znajek process can provide
the primary power in the jets.

A seminal idea that has been followed by many researchers in the field
is that of the magnetohydrodynamical model of
Blandford & Payne
(1982).
These authors proposed that the angular momentum of a magnetized accretion
disk around the collapsed object is the responsible for the acceleration
of the plasma. The magnetic field lines are taken to be frozen into the disk
and the plasma is assumed to follow them like a "bead on a wire", at least
close to the disk. If the field line forms an angle with the plane of the
disk smaller than 60°, the displacements of the plasma from its
equilibrium
position become unstable. This happens because along these field lines the
component of the centrifugal force will be larger than the component of the
gravitational force and the plasma will be accelerated outwards. Then, in
its origin, the outflow motion has an important "equatorial" component,
while on larger scales the jets are observed to have a motion that is
dominantly "poloidal". In other words, after the acceleration a
collimating mechanism is required to change the wide-angle centrifugal
outflow into a collimated jet.

This collimation is proposed to be achieved as follows.
Inside an inner region, the magnetic field energy density is larger than
the kinetic energy density of the flow but at some distance from the disk
(the Alfvén surface), this situation reverses and the flow stops
corotating
with the disk. This causes a loop of toroidal (azimuthal) field to be added
to the flow for each rotation of the footpoint of the field line. The
tension of this wound-up toroidal field that is formed external to the
Alfvén surface produces a force directed toward the axis (the "hoop
stress") that eventually
collimates the flow into a jet. Most models for the production of jets in
the astrophysical context use elements of MHD acceleration and collimation.

Recently, several groups
(Spruit et al 1997,
Lucek & Bell
1997,
Begelman 1998)
have pointed out that the toroidal field traditionally held responsible for
collimating jets in the MHD mechanism is unstable and cannot collimate the
jets effectively. It has been proposed alternatively that the collimating
agent is the poloidal component of the magnetic field.

Koide et al (1998)
have performed for the first time full general relativistic MHD numerical
simulations of the formation of jets near a black hole. Their results
suggest that the ejected jet has a two-layer structure with an inner,
fast gas-pressure driven component and an outer, slow magnetically
driven component. The presence of the inner, fast gas-pressure driven
component is a result of the strong pressure increase produced by shocks
in the disk through fast advection flows
inside the last stable orbit around a black hole. This feature is not seen
in non-relativistic calculations.

Within the uncertainties of the small sample, the velocity
of the jets seems to show a bimodal distribution, with some sources having
vjet 0.3c and
others having vjet 0.9c. Two explanations have
been offered in the literature. On one hand,
Kudoh & Shibata
(1995)
suggest that the terminal velocity of the jet is of order of the Keplerian
velocity at the footpoint of the jets, that is, that the fastest jets
probably come from the deepest gravitational wells
(Livio 1997).
However, recent observations suggest that Sco X-1 which is a neutron
star binary has vjet ~ 0.5c
(Fomalont 1999),
departing from the bimodal distribution. On the other hand,
Meier et al (1997)
propose that the velocity of the jets is regulated by a magnetic "switch",
with highly relativistic velocities achieved only above a critical value
of the ratio of the Alfvén velocity to the escape velocity. The
determination of the mass of the collapsed object in a larger number of
jet sources would discriminate between these two models.

While it seems that a steady state MHD model can account
for the formation of continuous relativistic jets, the events discussed by
Mirabel et al (1998),
Belloni et al (1998), and
Fender & Pooley
(1998)
that seem to involve a connection between the disappearance of the inner
accretion disk and the sudden ejection of condensations may require a
different
mechanism. Clearly, the time seems to be ripe for new theoretical advances
on the models of formation of relativistic jets that take into account the
observational features found in stellar jets.

Another characteristic that the jet models must account
for is the production of relativistic particles that will produce the
synchrotron emission that is observed in several sources. As in other
astrophysical contexts, it is believed that the acceleration of
electrons to relativistic speeds takes place in shocks
(Blandford & Ostriker
1978).
On the other hand, most of the X-ray binaries are "radio-quiet", implying
that relativistic electrons and/or magnetic fields are not always present
in sufficient amounts.